1. Field of the Invention
The invention relates to an amplifying circuit for use with a transducer. The present circuit is also particularly useful for amplification of a signal from a high impedance source.
2. Description of the Prior Art
The use of field-effect transistors for amplification in low frequency circuits has been well accepted since such transistors have a long life, a high degree of reliability and they are generally readily available commercially. Therefore, such field-effect transistors are utilized with electrostatic type transducers for preamplifiers. The usual circuit configuration for this type of preamplifier is shown in FIG. 1.
Referring now to FIG. 1 which is a circuit diagram of a typical prior art circuit for use with an electrostatic type microphone, a field-effect transistor 10, which is an N-channel junction type field-effect transistor in this example, is connected at its drain electrode to a DC power source +V.sub.DD and at its source electrode to the circuit ground through a series circuit of resistors R.sub.1 and R.sub.2. A gate biasing resistor R.sub.G is connected between a connecting mid-point of the resistors R.sub.1 and R.sub.2 and the gate electrode. An electrostatic type mechanical-electrical transducer 12 having electrostatic capacity of, for example, about 10 to 100 PF is connected between the gate electrode, that is, an input terminal t.sub.1a and a ground terminal t.sub.1b. A matching transformer 13 is provided in such a manner that its primary winding 13a is connected by one lead to the source electrode of the transistor 10 through a blocking capacitor 14 and by its other lead to the circuit ground. Its secondary winding 13b is connected by its leads to separate output terminals t.sub.2a and t.sub.2b. Reference numeral t.sub.2c designates another output terminal which is grounded. Such a circuit is called a source follower type amplifier.
It is desirable that the resistance value of the gate biasing resistor R.sub.G of such a preamplifier be relatively large to increase the input impedance of the preamplifier to more than 500 meg-ohms, for example, and to decrease the effect of noise generated from the resistor R.sub.G. In fact, the larger the value of this resistor R.sub.G is, the better the tone becomes.
This might suggest that the resistance of this resistor R.sub.G should be made infinity, that is, that the resistor R.sub.G should not be used. While this idea will satisfy the above described requirement, the DC operating point of the gate becomes unstable and the dynamic range of the preamplifier becomes limited.
The above problems will be further described with reference to FIGS. 2 and 3. FIG. 2 is a circuit diagram for use in explaining an amplifier having a source follower type field-effect transistor, in which reference character R.sub.3 indicates a source resistor corresponding to the resistors R.sub.1 and R.sub.2 of FIG. 1 which are connected in series. Reference character V.sub.DD expresses the power source voltage, V.sub.DS the voltage between the drain and source, V.sub.R the voltage across the source resistor R.sub.3 (output voltage), V.sub.GS the voltage between the gate and the source, V.sub.G the gate DC voltage and I.sub.D the drain current, respectively.
FIG. 3 is a diagram showing the V.sub.R - I.sub.D operating characteristic curves of the source follower type field-effect transistor amplifier of FIG. 2 when V.sub.GS is taken as parameter. Reference numerals 15, 16 and 17 indicate V.sub.R - I.sub.D curves at the conditions V.sub.GS = 0, V.sub.GS = -V.sub.GS1 (V.sub.GS1 / 0) and V.sub.GS = -V.sub.GS2 (V.sub.GS2 &gt; V.sub.GS1 &gt; 0), respectively. Reference numeral 18 identifies a load line (I.sub.D = V.sub.R /R.sub.3).
In FIG. 3, for example, a point A shows the operating point in the case when the condition V.sub.GS = -V.sub.GS1 is satisfied, and hence the output voltage V.sub.R is made equal to V.sub.G -(-V.sub.GS1) and becomes different from the gate DC voltage V.sub.G by the absolute value of V.sub.GS1. Generally, in a junction type field-effect transistor, if the gate and source are biased therebetween in a forward direction, the input impedance is rapidly lowered and as a result, in order to increase the input impedance, the gate and source must be biased therebetween in a reverse direction. Accordingly, the operating point must be selected on the load line 18 between the origin O and an intersection B (V.sub.R = RI.sub.DSS and I.sub.D = I.sub.DSS) of the curve 15 and the load line 18. It will be noticed from the above description that the upper limit of the output voltage V.sub.R is the voltage RI.sub.DSS and when its dynamic range is required to be wide, it is necessary to increase the voltage RI.sub.DSS approaching the power source voltage V.sub.DD as much as possible. The operating range of the output voltage V.sub.R is from zero to RI.sub.DSS as described above and an input signal applied to the field-effect transistor 10, which is equivalent to an output of an electrostatic type mechanical-electrical transducer, has such amplitude that the positive and negative parts thereof are substantially equal to each other with respect to the operating point. Therefore, if the operating point is selected so that the condition V.sub.R = RI.sub.DSS /2 is satisfied, the dynamic range is made the largest.
Consequently, in the prior art source follower type field-effect transistor amplifier, if the dynamic range is designed to be wide as much as possible, a gate biasing resistor for reversely biasing the gate of the field-effect transistor 10 with respect to the source thereof may be required.
On the other hand, the gate biasing resistor will cause tone deterioration due to the noise generated therein and a lowering of the input impedance. Further, the difference of V.sub.GS taken between the input and output voltages and the voltage V.sub.GS taken between the gate and the source changes substantially with a square characteristic according to the drain current I.sub.D. As a result, the voltage V.sub.GS is increased when the current I.sub.D is small while the former is decreased when the latter is large under the reversely biased condition. Accordingly, as shown in FIG. 3, the output voltage V.sub.R is compressed in waveform at the region where the voltage V.sub.G is low with the result that the distortion becomes large.
The above description was made with respect to a junction type field-effect transistor. However, even in the case of a MOS type field-effect transistor, a gate biasing resistor is required for wide dynamic range.